The invention concerns a hydraulic bolt tensioning device for tightening and re-leasing highly loaded screws, comprising a cylinder that is supported on a machine part to be clamped by means of a threaded bolt and a nut; an exchangeable bushing that can be screwed onto a free threaded end of the threaded bolt; at least one pressure-loadable tensioning piston that is seal-tightly guided in the cylinder, is supported on an extension of the exchangeable bushing that is screwed onto the threaded end of the threaded bolt projecting past the nut, and forms with the cylinder an annular space; a pressure medium supply; an adjusting device for adjusting a nominal pressure corresponding to a nominal pretension; a device for pressure relief of the bolt tensioning device; and a pressure sensor.
Such a hydraulic bolt tensioning device is disclosed in U.S. Pat. No. 7,469,592 (US 2008/0173140) of instant applicant.
Bolt tensioning devices have the task of applying a precisely predetermined initial tightening force onto a threaded bolt in order to tighten or release the nut screwed onto the threaded bolt. For this purpose, an exchangeable bushing of the bolt tensioning device is screwed onto the projecting thread above a nut and then the bolt tensioning device is subjected to hydraulic pressure. This has the result that the threaded bolt is tensioned by being longitudinally expanded. The forces that are applied onto the threaded bolt are extremely high so that the threaded bolt and the bolt tensioning device and its individual parts are exposed to great loads. In order to prevent that these highly loaded individual parts upon tensioning of a threaded bolt will break because, as a result of surpassing the load changes that are absorbable by these parts, a fatigue fracture will occur, it is desired to exchange the parts that are at risk of fatigue fracture before such a fatigue fracture can occur. This is also true for the premature exchange of other wear parts, for example, seals. This is achieved by a so-called load-cycling counter that is switched for each actuation of pressure loading of the bolt tensioning device, wherein counting of idle strokes is prevented in that the load-cycling counter switches only above a predeterminable pressure. Such a load-cycling counter is disclosed in U.S. Pat. No. 5,682,025 of the instant applicant.
A further safety measure is disclosed in DE 43 41 707 A1 of the instant applicant that becomes effective when the exchangeable bushing that is in particular highly loaded in these hydraulic bolt tensioning devices, after a certain number of screw expansions, i.e., load changes in the area of the threaded bolt end, will tear off in individual cases before the calculated load changes have been reached.
Since in such a case the entire system will be relieved suddenly, the torn-off part of the exchangeable bushing, together with the piston and the cylinder, can be thrown off at significant energy and significant damage on neighboring machine parts as well as injuries to the operator may be caused. In order to prevent this, in DE 43 41 707 A1 of the instant applicant a catching device for these parts of the bolt tensioning device is provided.
The load-cycling counter disclosed in U.S. Pat. No. 5,682,025 as well as the hydraulic bolt tensioning device disclosed in DE 43 41 707 A1 with catching device have found acceptance in practice but these devices do not protect against tearing off of the threaded bolt to be tensioned which can occur in particular when the threaded bolt is already damaged by over expansion, by a defective thread, or by material defects.